1. Field of the Invention
Our invention relates to battery powered lighting devices, as flashlights, emergency lights, and the like, having solid state semiconductor photo-emitters, typically multiple light emitting diodes, as the lighting source. Our invention further relates to an LED (light emitting diode) series array for use, for example, in lighting devices.
2. Background
High power light emitting diode (“LED”) portable lights, for example flashlights, emergency lights, cave lights, and the like are gaining market share. Traditional light bulbs produce light by heating a filament to its incandescence temperature. This is wasteful of energy, especially stored energy, because as much as four fifths of the light is lost as ohmic heating, that is, I2R heating, and only one fifth of the energy into light. By way of contrast, light emitting diodes do not rely on heating a filament to incandescence, but on carrier injection. Thus, LEDs have much less energy loss through incandescent heating. As a result they are more efficient then incandescent lights.
A further advantage of LEDs is that they are long lived. An LED will last from 10,000 hours to 100,000 hours or more. Additionally, LEDs are encased in high strength, optical grade polymers, such as optical grade epoxy or silicone resins. Without a glass or filament to break, LEDs are desirable for hostile environments.
Previously, LEDs did not produce enough light for true flashlight or emergency light use. However, new LED products are entering the marketplace, and these new products provide high illumination.
In a conventional LED array, a plurality of LEDs (which individually emit individual light beams of bandgap determined wavelengths) are arranged in a line substrate. The light beams from the individual LEDs are converged by a lens, as a fresnel lens or a rod lens. The lens is placed at a fixed spacing from the LEDs, so as to provide the desired illumination.
While white light is desired, it is not emitted by semiconductor light emitting diodes. In the LED array of this type, one LED may emit green light of a wavelength of 555 nm, may be used in conjunction with an LED which emits yellowish-green light of a wavelength of 565 nm. These LEDs may be used with LED, which is capable of emitting red light of the wavelength of 635 nm has.
When the above-mentioned LEDs for emitting red light, which is reflected by the red portions because of its wavelength, is used in the LED array, the red portions in the original reflect the red light, so that the image sensor is not capable of reading the red portions. Thus, when the LED array is provided with the LEDs for emitting green or yellowish green light and the LEDs for emitting red light, the subject is irradiated with red light, as well as green or yellowish-green light, so that the subject appears to be lit by white light.
However, in order to obtain an LED array in which different types of LEDs of different wavelengths are used to emit light beams at wavelengths at these different wavelengths in order to additively produce white light a very large number of lead wires are necessary, resulting in an LED array with complicated wiring that is expensive to manufacture.
Another problem with the new, high power LED flashlights is that blue LEDs, which are required to produce white light, have a forward voltage of 3.3 to 4.0 volts, and typically about 3.5 volts. The design issue is that most consumer batteries have a cell voltage of 1.35 to 1.50 volt nominal. This means that three batteries must be used in an LED flashlight. This is an output of 4.05 to 4.50 volts to produce white light. This voltage level, 4.05 to 4.50 volts cannot be directly applied to a 3.3 to 4.0 volt LED. The high voltage will damage the LED, and significantly shorten its life.
In order to overcome this problem, a current limiting resistor has heretofore been proposed, dropping about 1.00 volt. This is about 18 to 22 percent of the battery's power, and represents significant waste; especially where portability and long time between battery recharges is desired.
Moreover, in order to use the energy stored in the batteries more efficiently, certain efficiencies are obtained by operating series connected LEDs at still higher voltages. For example, with an LED series circuit having LEDs whose emissions add up to white light, a series circuit of eight LEDs can be operated to give white light at an applied voltage of 28 volts.
Since the response time of a solid state lighting device is on the order of nanoseconds, while the human eye does not perceive flicker at frequency approaching and above 100 hertz, the power supply can operate with a short duty cycle, for example, as low as about ten percent, with short, high current pulses, at high electrical efficiency.
Thus, a clear need exists for a low cost “white light” light emitting diode array that is characterized by a high degree of manufacturability, for use in a solid state lighting device. The solid state lighting device requires a step up power supply, preferably operating in a pulse mode, at nanosecond level pulses.